1. Field of the Invention
The present invention relates to a process for the production of steel in a basic converter employing liquid converter slag.
2. Brief Description of the Background of the Invention Including Prior Art
Usually fired or tar-bonded bricks based on dolomitic (MgO+CaO) or magnesitic (MgO) materials as well as combinations of both are employed for basic linings of converters. These linings are subjected to strong chemical, thermal and mechanical wear. The mechanical wear is caused by the erosion of the starting materials and by the use of scrap and pig iron. Thermal wear is based on the sensitivity of the refractory ceramic bricks against temperature variations. Mainly, however, the life time of the converter lining is influenced by chemical causes, that is by the attack of slag against the basic lining.
It is known that the chemical wear of the lining depends on the composition of the slag during the converter process. According to the state of the art a high contents of iron oxide results in a negative influence on the process especially at the prevailing high temperatures, since a reaction between the iron oxide and the carbon of the binder of the refractory lining occurs and thereby the lining is decarburized. Further, it is known that increasing silicon contents in the slag decrease the lifetime of the basic refractory materials. However, in particular, during the first minutes of the converter process there are formed high silicon dioxide concentrations and an acid slag, based on the silicon contents of the pig iron and on the corresponding iron oxide, since the lime employed does not dissolve this rapidly.
Low-viscosity iron oxide silicate slags of low basicity behave aggressively against the basic lining, they penetrate during the blowing process into the pores of the bricks and react there with the calcium oxide of the dolomite. In addition, the slag is characterized by a considerable capacity to dissolve magnesium oxide. This capacity of the slag to dissolve is largest at the beginning of the converter process and decreases toward the end of the blowing with increasing basicity. Therefor, the attack of the slag on the converter lining is largest at the beginning of the converter process.
Therefor, steps for increasing the stability have to be directed to increase the basicity of the slags in particular in the starting phases of the converter process. A chemical-metallurgical attack of the slag onto the refractory, basic converter lining can only be reduced by way of a saturation of the slag in magnesium oxide, calcium oxide and respectively dicalcium silicate during the total converter process. The composition of the slags at the end of the converter process cannot solely be used as a criterion for the wear of the refractory material. A saturation of the initial slag with magnesium oxide and respectively calcium oxide encountered hitherto large difficulties, since the added magnesium oxide and respectively calcium oxide did not dissolve sufficiently rapidly. The solubility improves only during the converter process.
An increase in the speed of dissolution of magnesium oxide as an added flux charge material in a magnesitic lines converter has been achieved according to German Open-laid Disclosure Document DE-OS 28 52 248 in the following way in particular at the beginning of the converter process under maintaining of the magnesium oxide saturation during the total converter process and under avoidance of fluxing agent additions. The magnesium oxide or the magnesium oxide containing materials are blown into the zone of combustion in the presence of calcium oxide or of calcium oxide containing basic materials at the beginning of the converter process. For this purpose precisely defined amounts of flux materials with certain grain sizes are employed. Then the required balance of the amount of calcium oxide is added. This process results in a rapid dissolution of the flux charge materials based on the high temperature in the region of the combustion zone. However, the high magnesium oxide contents required for a protection of the converter lining are reached only after a certain reaction time, which can be short. After about 20 percent of the converter process time a magnesium oxide saturated slag is present. In addition, a blow lance is required for blowing in the flux charge materials, which blow lance has to be suitable for blowing in of solids. According to this process, the initial slag is not saturated in magnesium oxide. Furthermore, the basicity of the initial slag is so low as is known from the classic LD (Linz-Donawitz) process.
In addition the increase of the basicity of the initial slag and thereby the decrease of the magnesium oxide and calcium oxide contents required for saturation can be achieved by employing materials with high basicity and low melting point, such as for example converter slag, before or after the starting of the converter process.
The use of converter slag is known and for example has been taught in the French Pat. FR-PS 1,509,342. This patent discloses a process for converting of pig iron by use of liquid converter slag. It is characterizing for this process that the required slag additions (lime and silicon dioxide flux charge) have to be entered in a granulated form in order to avoid spittings during the bringing in of the pig iron and that the converter has to be rotatable around its longitudinal axis in a horizontal position. This is a special variant of the LDAC-process, where the final slag always remains in the converter and the slag is tapped after about 50 percent of the blowing time.
The use of liquid converter slag is also known from "Steel in the USSR" of August 1972, pages 608 to 611 (Kusnetsov and others). In this case a retention of from 20 to 25 percent of the amount of the slag of the previous melt is employed for acceleration of the slag formation and for increasing the basicity. The slag is thickened with lime and the total scrap is charged. Then the pig iron is charged. The slag is rendered inactive. An operation with larger amounts of slag is technically not mastered based on the occurrence of spittings.
Also, in "Metallurg" 9, 1975, pages 18 to 20 (Kusnetzsov and others) the use of retained slag is described. This slag is stripped before the filling with pig iron by way of lime and scrap. Also in this case, it is only possible to technically control the blowing with a maximum of 50 percent of the slag remaining in the converter.
Further, it is known to re-employ solid, reworked converter slag (Revue de Metallurgie, May 1978, pages 297 to 301, author R. Ando and "Fachberichte Huettenpraxis Metallweiterverarbeitung", October 1978, pages 789 to 796, author H. Nashiwa and others). The preferred starting amount of solid slag amounts of 25 kilograms per ton of steel and is charged after three blowing minutes with a chute. Dolomite in an amount of 30 kilograms per ton of steel is charged in order to avoid spittings during the middle phase of the converter process (with 18 percent of magnesium oxide contained in the dolomite). With such a manner of processing the slag cannot be saturated during the total time of the converter process, only the final slag reaches a saturation in magnesium oxide. However, the main wear of the converter occurs just at the time of the beginning of blowing caused by slag of low basicity and of a high capacity to dissolve magnesium oxide. This operation process is performed in Japan, in particular in order to decrease the industrial waste (in this case the LD-slags).